Proper control of the immune system is critical to human health. A hypersensitive immune system can lead to allergies and autoimmune disease, whereas impairment of the immune system leaves an individual susceptible to infections and even cancers. Dendritic cells (DCs) play an important role in ensuring proper regulation of the immune system. In response to pathogens, DCs rapidly alter their phenotype to limit the spread of infection, and initiate an adaptive immune response. DCs are also involved in preventing autoimmunity by maintaining self tolerance. The critical role of DCs in both immunity and tolerance has made them important therapeutic targets. DCs can be used to enhance immune responses against pathogens and tumors, as well as to induce tolerance to self or even transplant antigens. Because of their importance to health and disease, as well as their therapeutic relevance, it is important to understand how DCs function. Here, we propose to investigate a new and little understood area of DC biology, microRNA (miRNA) regulation. miRNA are an emerging class of abundant, small, non-coding RNAs that function in a vast post-transcriptional regulatory network. miRNA bind to mRNAs with near-complementarity and prevent their translation, thereby helping to control the molecular regulation of the cell. We hypothesize that miRNA regulation plays an important role in DC function. To test our hypothesis, we will develop a novel platform for stably and specifically inhibiting the activity of a miRNA. Lentiviral vectors will be constructed to express supraphysiological levels of a miRNA target sequence, which will act as a competitive inhibitor, or decoy, of the miRNA. By introducing decoy vectors into mouse and human DCs, we will be able to interfere with the ability of a specific miRNA to regulate its endogenous targets, and thereby impair its function. Through the introduction of decoy vectors, we will effectively and individually inhibit a panel of miRNAs, and monitor the subsequent ability of the cells to respond to infectious and inflammatory stimuli, produce cytokines, upregulate co-stimulatory molecules, home to lymph nodes, and finally, activate an antigen-specific T-cell response. Using this screen, we will be able to identify those miRNAs with the most dramatic impact on DC function. By analyzing RNA and protein expression patterns in miRNA 'knocked down'DCs, we will reveal the identity of genes targeted by miRNA, thereby enabling us to understand novel and previously unappreciated mechanisms of functional regulation in DCs. Our investigations into the miRNA-mediated regulation of mouse and human DC function will identify potential new causes of immune dysfunction, and supply new factors which may be targeted to either enhance or subdue an immune response. PUBLIC HEALTH RELEVANCE: Understanding how the immune system is regulated is critical for developing new strategies to enhance or subdue immune responses. Here, we will investigate a newly discovered form of gene regulation, called microRNA regulation, to determine its impact on the control of the immune system. This could lead to improved vaccine design, and better management of inflammation and autoimmunity.